The present invention relates to golf club heads and particularly to golf club heads having impact surface portions fabricated from shape memory alloys.
U.S. Pat. No. 5,176,384 to Sata et al. discloses a golf club head wherein the impact surface of the club head is fabricated from a thermoelastic type martensite transformation alloy, such as a NiTi-based or a copper-based alloy, which is said to have improved carry and directional stability by reason of the use of a thin plate as an impact surface and because of the stress-induced martensite elastic characteristics of the material.
Pending U.S. patent application Ser. No. 08/760,251, the disclosure of which is incorporated herein by reference, discloses a golf club head having an impact surface formed of "pixels", which may comprise shape memory alloy. In one embodiment pixels comprise wires arranged in an array, with wire ends forming the impact surface. Alternately, the pixels may comprise portions of the impact surface that have received different treatment, such as annealing. This prior application indicates that when using a shape memory alloy material, such as NiTi-based alloy, the pixel wires may be in a "superelastic" or "optimized elastic" condition. It is additionally stated that the pixels may be a mixture of superelastic and Martensitic material.
The above discussed disclosures do not address the temperature variation of the mechanical characteristics of shape memory alloys, but assume that the properties remain constant over the usual range of playing temperature, which is about zero to +40 degrees C.
It is generally observed that the performance and impact characteristics of golf balls change with playing temperature. At colder temperatures, for example in the range of zero to 10 degrees C., golf balls tend to have stiffer characteristics. When a ball is struck with a club, the impact will result in vibrations of different frequencies and different amplitudes in the club shaft because of the ball stiffness, which affects the flight of the ball. The stiffness of a conventional club head stays constant throughout the temperature range. At warmer playing temperatures, for example in the range of 30 to 40 degrees C., the ball tends to be more easily deformed upon impact, and different shaft vibrations will occur when the ball is struck by the club head.
The inventors have concluded that it would be desirable to provide a club head having an impact surface whose properties vary with temperature in a manner that complements the temperature variations of the golf ball and allows for a relatively constant interaction between the club head and the ball irrespective of the temperature. Accordingly, at cold temperature the club head impact surface would be softer, to offset the relatively stiffer ball. At higher temperatures, the impact surface would be stiffer, to offset the relatively softer ball. A temperature controlled club head would allow a golfer to have the same feel and club head-ball interaction on cold days and warm days.
In U.S. Pat. Nos. 4,772,112 and 4,896,955 there is discussed the temperature variation of mechanical properties of NiTi based shape memory allows, and particularly the use of the "optimized elastic" properties of the alloy for application to eyeglass frames. In those patents the stated objective was to provide frame components that exhibit elastic properties over a wide temperature range of -20 to +40 degrees C. The material is also capable of exhibiting shape memory characteristics, in addition to elastic characteristics, at the lower temperatures within that range. The object of these patents is to allow bending of an eyeglass frame at different temperatures and allow it to return to a usable shape without permanent deformation.
It is an object of the present invention to provide a golf club head with an impact surface that has different and desirable mechanical characteristics at different playing temperatures, to thereby provide a softer impact associated with the Martensitic phase at lower temperatures and to provide a stiffer impact associated with the austenitic phase at higher temperatures which reduces vibrations and provides a more constant feel for the club.